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February 3, 2011

Frozen San Dunes On Mars Showing Activity

by Sam Savage

According to research using images from a NASA orbiter, sand dunes in a vast area of northern Mars long thought to be frozen in time are changing with both sudden and gradual motions.

NASA said on Thursday that the dune fields cover an area the size of Texas in a band around the planet at the edge of Mars' north polar cap. The new findings suggest they are among the most active landscapes on Mars.

However, few changes in these dark-toned dunes had been detected before a campaign of repeated imaging by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.

HiRISE Deputy Principal Investigator Candice Hansen of the Planetary Science Institute said that scientists considered the dunes to be fairly static, shaped long ago when winds on the planet's surface were much stronger than those seen today. However, several sets of before-and-after images from HiRISE over a period covering two Martian years tell a different story.

"The numbers and scale of the changes have been really surprising," Hansen said in a statement.

The researchers report identifies the seasonal coming and going of carbon-dioxide ice as one agent of change, and stronger-than-expected wind gusts as another.

"This gas flow destabilizes the sand on Mars' sand dunes, causing sand avalanches and creating new alcoves, gullies and sand aprons on Martian dunes," she said. "The level of erosion in just one Mars year was really astonishing. In some places, hundreds of cubic yards of sand have avalanched down the face of the dunes."

The researchers discovered that scars of past sand avalanches could be partially erased by wind in just one Mars year.

Models of Mars' atmosphere do not predict wind speeds adequate to lift sand grains, and data from Mars landers show high winds are rare.

"Perhaps polar weather is more conducive to high wind speeds," Hansen said in a press release.

Modifications were seen in about 40 percent of these far-northern monitoring sites over the two-Mars-year period of the study.

A report four months ago showed that those changes coincided with the time of year when ice builds up.

"The role of the carbon-dioxide ice is getting clearer," said Serina Diniega of NASA's Jet Propulsion Laboratory, Pasadena, Calif., lead author of the earlier report and a co-author of the new report. "In the south, we saw before-and-after changes and connected the timing with the carbon-dioxide ice. In the north, we're seeing more of the process of the seasonal changes and adding more evidence linking the changes with the carbon dioxide."

Researchers are using HiRISE to repeatedly photograph dunes at all latitudes, in order to understand winds in the current climate on Mars.

Hansen said "It's becoming clear that there are very active processes on Mars associated with the seasonal polar caps."

The new findings contribute to efforts to understand what features and landscapes on Mars can be explained by current processes, which require different environmental conditions.

"Understanding how Mars is changing today is a key first step to understanding basic planetary processes and how Mars changed over time," HiRISE Principal Investigator Alfred McEwen of the University of Arizona, Tucson, a co-author of both reports, said in a statement.

"There's lots of current activity in areas covered by seasonal carbon-dioxide frost, a process we don't see on Earth. It's important to understand the current effects of this unfamiliar process so we don't falsely associate them with different conditions in the past."

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Image 2: Three images of the same location taken at different times on Mars show seasonal activity causing sand avalanches and ripple changes on a Martian dune. Image credit: NASA/JPL-Caltech/University of Arizona